Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, [no address given] and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log

It turned out that the reason was line 11 in the REDMINE_DIR/public/.htaccess file:

Options +FollowSymLinks +ExecCGI

Commenting it solved the problem

regards

Related issues

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Defect #7256: redmine/public/.htaccess must be moved for non-fastcgi in...